Hyperpolarized helium is presently used in magnetometry for the detection and measurement of weak magnetic fields. A new application of hyperpolarized helium is the in vivo exploration of the respiratory airways in humans. The principle consists of the patient inhaling a gaseous mixture containing hyperpolarized helium, followed by performing magnetic resonance imaging (MRI) to visualize the pulmonary ventilation. The imaging can be performed in a conventional strong-field tomograph (&gt;1 tesla) or in a tomograph with a weaker field, possibly dedicated to that application. The inventors have published a report in the Academie des Sciences de Paris, Vol. 230, Series IIb, p. 671-700, 1997, describing various aspects of .sup.3 He gas NMR in the live lung as well as the equipment for the performance of an in vivo NMR. This type of application requires the production of hyperpolarized gas with a pressure that is sufficient to allow inhalation (P.gtoreq.1 bar).
Also known in the art are various processes and installations for the production of hyperpolarized helium. The general principle is based on optical laser pumping. As an example, French patent FR 8914894 describes an atomic or molecular vapor cell for optical pumping according to the prior art. The gas from this cell is constituted of a mixture of helium-3 and helium-4. However, the apparatus of the prior art only allows polarization of helium gas at low pressure, which then must be compressed at the outlet of the cell. Compression without loss of polarization of the helium gas is delicate and requires complex or expensive equipment.
The prior art also discloses two solutions for compressing the hyperpolarized helium: a mechanical compression technique using a nondepolarizing device developed by the group of Professor E. Otten of the University of Mainz and a cryogenic compression technique described in French patent FR 9601973.
FR '973 concerns an installation for the production of polarized helium-3, comprising a storage reservoir of helium-3 in liquid phase. The production of helium-3 with this type of installation requires an accumulation phase, an optical pumping phase and an evaporation phase enabling delivery of polarized helium-3.
Thus, it would be highly advantageous to resolve these drawbacks and difficulties relative to noteworthy compression by developing a process which avoids the use of sophisticated mechanical compressors and allows the production of hyperpolarized helium at high pressure with simple equipment.